A. Field of the Invention
This invention relates to ultraviolet radiation curable compositions suitable for use as optical coatings and in electrical insulation.
Manufacturers of photographic films have been attempting to perfect three-dimensional-effect color films for some time. One approach currently under investigation is to deposit a clear, lenticulated optical coating onto conventional photographic print material. The optical coating is responsible for the three dimensional effect and is usually composed of a cross-linked, ultraviolet-radiation-cured acrylic polymer supported by a transparent polyester base. The optical coating is typically 8-11 mils (0.008-0.011 inch) thick. Lenticulated optical coatings are described in J. Nims et al, "Three Dimensional Pictures And Method Of Composing Them," U.S. Pat. No. 3,852,787 (Dec. 3, 1974).
An optical coating should possess several characteristics; foremost among them is optical clarity. The optical coating must not be susceptible to yellowing or other discoloration prior to, during, and after exposure and developing of the color print material underlying the optical coating.
In addition to optical clarity, the optical coating must be sufficiently flexible so as not to crack when the film is bent in half. An elongation of from 15 to 65 percent is preferred.
The physical characteristics of the optical coating are highly dependent upon the choice of acrylate-terminated oligomers employed in the manufacture of the optical coating. In addition to providing a cured acrylic optical coating which satisfies the requirements discussed above, the uncured optical coating composition should possess a rapid cure rate, thereby allowing processing speeds of at least 15 feet per minute and preferably 30-60 feet per minute.
Typically, acrylate-terminated oligomers which possess polyester or polyether backbones have been employed in the manufacture of optical coatings. Polyester-based acrylic coatings display poor hydrolytic stability. Conventional water-based photographic developers can easily penetrate polyester-based, and to a lesser extent, polyether-based, acrylic optical coatings, thereby causing severe yellowing of the optical coating. Polyether-based acrylic coatings suffer from poor oxidative stability.
The ultraviolet radiation curable compositions of the present invention also have utility in the field of electrical insulation. Electrical insulation should possess several properties, such as a low dielectric constant, low dissipation factor, low moisture vapor transmission, and a high surface resistivity. The compositions of the present invention possess these properties.
B. Description of the Prior Art
H. Hisamatsu et al, "Photopolymerizable Isocyanate-35 Containing Prepolymers," U.S. Pat. No. 3,891,523 (June 24, 1975) discloses UV curable, acrylate-terminated polyurethane prepolymers which have a free isocyanate content of from 0.3 to 15 percent based on the total weight of the prepolymer and a cure initiator. The excess isocyanate groups are stated to impart superior adhesion to a substrate.
N. Miyata et al, "Photopolymerizable Vinylurethane Monomer," U.S. Pat. No. 3,907,865 (Sept. 23, 1975) discloses acrylate-terminated polyurethane oligomers which are highly UV curable and which, upon curing, form a non-yellowing acrylic film. The reference requires the use of xylylene diisocyanate in the preparation of its acrylate-terminated polyurethane oligomer.
D. Lorenz et al, "Radiation Curable Coating Composition Comprising An Acryl Urethane Oligomer And An Ultra-Violet Absorber," U.S. Pat. No. 4,129,667 (Dec. 12, 1978) discloses an acrylate-terminated, polyether or polyester-based polyurethane oligomer which is employed in combination with an acrylic ultraviolet radiation absorber.
D. Lorenz et al, "Radiation Curable Coating Composition Comprising An Acryl Urethane Oligomer And An Ultra-Violet Absorber," U.S. Pat. No. 4,135,007 (Jan. 17, 1979) discloses a radiation curable coating composition comprising an acrylate-terminated, polyether or polyester-based polyurethane oligomer and a benzylidene acid ester.
D. Carlick et al, "Isocyanate-Modified Esters," U.S. Pat. No. 3,783,151 (Jan. 1, 1974) discloses that the hydrophillic nature of polyester-based acrylic films may be somewhat lessened by introducing urethane linkages on the polyester monomer via an isocyanate. The urethane-branched, polyester-based acrylic films so prepared do not undergo color change under normal curing conditions.
A. Boranian et al, "Floor Tile Production And Products," U.S. Pat. No. 3,924,023 (Dec. 2, 1975) discloses mar-resistant transparent wear layers which are coated upon vinyl asbestos and asphalt floor tiles. A UV curable composition comprising an acrylate-terminated urethane oligomer of the formula: ##STR1## wherein A is either ##STR2## R being H or CH.sub.3 ; x is 0, 1 or 2; and B.sub.1, B.sub.2 and B.sub.3 are each taken from the group consisting of an alkylene of the type ##STR3## where R is H, CH.sub.3 or C.sub.2 H.sub.5 and y is either 0 or 1 to 6, cycloalkylene of the type ##STR4## wherein R is H, CH.sub.3 or C.sub.2 H.sub.5, x=0 or 1 or 2 and y=0 or 1 or 2, oxyalkylene of the type ##STR5## wherein R is H, CH.sub.3, or C.sub.2 H.sub.5, and x or y are both either 1, 2 or 3; ##STR6## where R is H, CH.sub.3 or C.sub.2 H.sub.5, y is 0, 1, 2 or 3, and x is 0, 1 or 2; and ##STR7## wherein R is H, CH.sub.3 or C.sub.2 H.sub.5, and x and y are both either 1 or 2, a reactive diluent, and a photoinitiator, is disclosed.
D. Kramm et al, "Radiation Curable, Ethylenically Unsaturated Thixotropic Agent And Method Of Preparation," U.S. Pat. No. 4,204,010 (May 20, 1980) discloses an acrylate terminated polyurethane oligomer containing an ester linkage which imparts thixotropic properties to liquid, radiation curable vehicles which are used as varnishes, coatings, photoresist adhesives, and sealants.